I've been to Nepal this year.
In general, it would be cheapest for you to fly to Delhi and than take a connecting flight to Nepal (it will cost you around $100-$120 depending on the airline).
The cheapest flight ticket I could find in the time I was going (which is roughly the start of the season there) was with Turkish Airlines. It cost approximately $870 and it went through Istanbul.
We then took a flight from Delhi to Kathmandu with Jet airways which cost about $120.
There are cheaper ways of getting from Delhi to Nepal by land using a train and a bus (which takes quite a while) or with a special Taxi that brings you to the border.
You should also check Royal Jordanian, El-Al (if this is the time of the year where they have direct flight to Delhi, they don't have it always), Turkish Airlines, Aerosvit (Ukranian) and Uzbekistan Airways. These are the relatively cheapest airline carriers that I know of.
Sometimes they even have deal if you are going for a relatively short period (1 month).
When you get to Delhi and you still want to get a flight from Delhi to Kathmandu I would recommend doing one of two things:
- If you have time, go to Delhi (probably the Main Bazar) and try to get a ticket from there it would be cheapest.
- If you don't have time try booking a ticket from Israel (we did that in Jet Airways).
The preferable airline carriers from Delhi to Kathmandu are (in that order):
- Sahara Air - They are the cheapest. I heard good things about them but it's sometimes hard to get tickets to them since they are so popular. Ticket should cost around $100.
- Jet Airways - They are not the cheapest but are very reliable. I flew with them. Ticket should cost around $120
- Royal Nepal - They are cheaper than Jet but they don't always fly and I've heard bad reviews about them not leaving on time and canceling flights. I flew with them back to Delhi and the flight was delayed for about 40min (which is actually very good for Royal Nepal, or so I'm told)
Have fun!